Furnace Won't Turn Off - Furnace Troubleshooting

Tim K.

22:17, 17 January 2015

29236

Those of you who read our articles regularly, know that we are a small, U.S. Veteran-Owned HVAC company in Southern California, and pride ourselves in giving people honest, straight answers to their questions. Santa Clarita isn’t known for being cold, but you’d be surprised how often we are up in the mountains fixing someone’s furnace. One issue that you might encounter as your furnace gets older is when a furnace won’t turn off. Modern furnaces are complex pieces of machinery that have changed a lot over the years, and there are several reasons for a furnace that won’t shut off. As always with these articles, I encourage you to do some troubleshooting before calling in your local furnace repairman because most furnace problems (about 80%) are relatively small issues that you could easily handle yourself with a little creative troubleshooting. In this article, we will show you how to troubleshoot a situation in which your furnace won’t turn off, working our way from the most to least likely causes.

Why Your Furnace Won’t Turn Off

This article is a little different and is less common than the other problems. It is important to note, that there are a variety of reasons that your furnace won’t turn off, many of which are outside of your control. Don’t freak out just yet though, because by reading this article I’d give you no less than a 50/50 shot that your furnace issue is something you can handle yourself. Let’s take a look at the most common causes for a furnace that won’t shut off:

First, Check Your Thermostat Setting.

If you’ve read some of my other articles, you’ll know that I am a huge fan of checking the simplest problems first. There is a reason for this – it works! I’m honestly not trying to insult your intelligence, but you’d be shocked at how many times we’ve gotten a call to come out when a furnace won’t turn off, only to find out that it was really a cold-war between the wife who likes it at 80 degrees and the husband who likes it at 68. Just check with your significant other first – that’s all I’m saying. Then check the following:

Check that your thermostat is set to heat (vs cool).

Check that it is in the “auto” or “on” mode of operation.

Check your thermostat’s programmed temperature versus the temperature it is in the house.

If your thermostat’s programmed temperature is lower than the temperature you have set, then this tells you that your furnace is continuously running to try and keep up with this demand. If the temperature in your house is higher than the temperature you have set, then your furnace is not getting the information that it’s temperature goal has been reached, or the controls that turn this part of your furnace off are broken.

If your furnace won’t turn off even after the requested temperature for your house has been reached, then go to section 2 below.

Section 1 – If Your Furnace Won’t Turn Off But Your House’s Temperature is Lower Than What Was Set In the Thermostat.

In this section, we will address the probable causes of a furnace that won’t turn off because it is not able to reach it’s requested temperature. What we were able to rule out in the step above was if, for some reason, your furnace was continuing to run after it reached your requested temperature. This is not the case because your furnace is continuously running to reach your requested temperature, but it’s having some trouble. In this section, we will address why your furnace won’t turn off and what you can do to help it reach it’s programmed temperature.

1. Ensure That Your Requested Temperature is Reasonable.

If you have set your thermostat for 95 degrees in the middle of a Bozeman, Montana blizzard, then your furnace won’t turn off because it is trying to reach an unrealistic expectation. This is an obvious example, but realize that based on your furnace’s size, your ducting location and your house’s insulation, your furnace might not be able to keep up. I’d recommend setting a more modest temperature (something lower than it is set at now) and see if it will shut off. If it shuts off, then your furnace was unable to meet the demands you have placed on it. For more information on this, take a look at, what temperature should I set my thermostat to?

2. Check Your Furnace Air Filter.

Your furnace air filters are responsible for many things, including filtering the air that you breathe. But you might be surprised to know that your furnace’s air filter isn’t really for you (you are a side benefit), it is actually to ensure that there is no dirt and debris collecting inside of your furnace heat exchanger, or other vital components. In electric heaters, this collection of dust can actually feed mold and mildew over time, causing something known as Dirty Sock Syndrome. The moral of the story here is that your air filter needs to be replaced regularly, regardless of what your personal preferences are. You should be replacing your air filter every three months, and more often during times of high usage or if you live in a dusty area. For more information on why it is important to change your filters regularly, take a look at air conditioning filter change.

If your filter is clogged up from infrequent changes, the restricted air flow limits your furnace’s ability to effectively warm your house. Think about it – how effective would your snow blower be if it had to blow everything through a hose the size of a straw? Bad analogy? Sure, but you get the idea. Try replacing your furnace air filter and see if this improves air flow. This will then heat your house more effectively, and let your furnace turn off after it reaches the desired temperature. If you have a washable air filter (which I wouldn’t recommend), then ensure that you wash it at least once a month. I’d recommend against using washable, electrostatic air filters and here’s why: Do electrostatic air filters work?

3. Check Your Ductwork for Leaks.

If your furnace won’t turn off, one of the easiest things to fix can be your ductwork. Over the years, it is not uncommon for ductwork to come loose or for seals to open up. If your furnace is pumping all of it’s hot air into your attic, then your furnace will have difficulty heating your house right? Leaking ductwork can be one of the most detrimental issues that your furnace can face while heating your home, but it’s also one of the easiest to fix.

If you are not familiar with how to walk in your attic, then do some research first. Only walk on joists, the supporting cross beams of your roof. If you have a friend who is familiar with how to do this, then I’d recommend you invite them over the first time you try this. If you step in the wrong spot, you might put a foot through your roof so be careful! Check the entire length of your ductwork while your furnace is running. This will make leaks easier to see. Seal any leaks with HVAC tape and reattach any detached segments.

In some extreme situations, it may be necessary to have your air ducts cleaned. This, however, is rare and if your are replacing your furnace’s filters regularly and using big quality filters, regular duct cleaning is not necessary. For more on this, try: should I have my air ducts cleaned? The answer to this is usually no, but it’s still worth a quick read.

Section 2 – If Your Furnace Won’t Turn Off After Your Set Temperature Has Been Reached.

If your furnace won’t turn off even after your house is at or above your programmed temperature, then the problem is usually more complex. Don’t go calling your furnace repairman just yet, though. Here are still a few things that you can figure out yourself. Some are within your control, and some are not.

1. Check Your Furnace Blower Motor.

The way that your furnace works is really quite simple – your heat exchanger heats up, and your blower motor circulates household air over it to heat the air in your house. You can think of your blower motor as kind of like a big fan (actually, that’s exactly what it is). Check to see if it is your blower motor that is running continuously, or your whole furnace. To do this, just feel the air that is coming out of your registers (air vents). If it is hot, then this is not your problem so move on. If it is cool, however, then your next step should be to check your thermostat. If your thermostat’s fan is set to “on,” then this is likely your problem. Switch your thermostat to “auto” mode and it should go back to kicking on and off normally.

If that doesn’t work, another possible problem could be the limit switch for your blower, but this would need to be serviced by an HVAC technician.

2. Check Your Thermostat.

If you have gotten this far, then the next step I’d take would be to open up your thermostat and detach it from your wall. Check to make sure that all of the wiring is attached to the receptacles on the back of your thermostat. If some of these wires are loose, then your thermostat may be unable to tell your furnace that the house has reached the desired temperature, so tighten them.

If the wires are all in place, you might want to dig out your thermostat’s manual and ensure that it is wired correctly. This can be frustrating, but take your time and you can figure it out. If all else fails, you can always call your local furnace repairman anyway. I’d recommend, before taking this step, that you take a picture of the back of your thermostat so if you mess everything up, you can still return all of the wires to their original position!

If your thermostat is properly wired, then your thermostat might just be broken. Thermostats are relatively cheap, and can range from $50 – $250. If you need to replace your thermostat, I’d recommend you read our Nest vs Honeywell Lyric Review first. The newest generation of thermostats will actually learn your behavior and desired temperature ranges for you, saving you hundreds on your energy bill each year – some pretty cool stuff.

Click to Enlarge – Excerpt Page 3 of 12 from the HVAC Design & Consultation Program

Don’t Overpay for Your New Furnace

Look, if you’ve tried all of these steps, and you still can’t repair it, then it just might be time. Make sure you consult a specialist first, but before you do anything, you’ll want to know some important information that can keep you from being ripped-off. Here’s a video on how to know a fair price when you see one:

Final Thoughts on Troubleshooting Your Furnace That Won’t Turn On

If you’ve made it to this point, nothing has helped and your furnace won’t turn off, then it’s just time to pick up the phone and call your local HVAC expert. Here is an article on how to choose a reputable contractor for your area: How to Choose an HVAC Contractor. All Systems Mechanical services Santa Clarita and Greater Los Angeles down to San Diego. If you live in our area and have any questions on related topics, please feel free to give us a call, just click the link below:

The University of Virginia - 2005 / The Wharton School of Finance - 2016 / U.S. Naval Aviator 2005-2015.
At All Systems Mechanical air conditioning and heating, we believe that the experience our clients have is every bit as important as the products they receive. Simply put, our results speak for themselves, and we'd be happy to help.
If you're in the market for a new AC or furnace, make sure that you get a fair price! Try our online calculator; click the tab on the top of this page for more information.

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